Electric drives for trucks and heavy goods vehicles

Most of the focus today appears to be on electric passenger vehicles (EPV), but there is also a significant shift in transport and haulage towards electric vehicles. Developments and advances in the EPV sector have been employed in heavy goods vehicles (HGVs) with marked success.

A range of trucks is available with various hybrid and full electric drive options. Electric vehicles such as fork lift trucks, mine trains and other traction and transport vehicles have been used in industry for many years but have suffered limitations due to both the technology and drive components used. Advances in battery technology, including battery management and charging devices originating in the EPV sector, are finding their way into HGVs with very good results, extending the capabilities of traditional and expanding the range of IEVs available.

Diesel powered HGVs are a major source of air pollution in the urban environment, and battery powered light- and heavy goods vehicles are seen as a solution to this problem. Li-ion batteries serve as the major energy storage component replacement. Fuel cells based on hydrogen are also a consideration, but are far less popular, possibly because while they offer larger capacities than batteries, they require more complex refuelling and controls.

Battery electric vehicles (BEVs) are not widely accepted by the industry now, because of the high price, limited driving range and long charging time. These limitations tend to make drivers more concerned about whether they can reach the destination based on the current state of battery. This factor, dubbed “range anxiety” has been identified as one of the main obstacles for the expansion of BEVs [3].

Electric goods vehicle types

There are four classes of electric truck or goods vehicle:

Catenary mains power plus battery (E-haulage)

Plug–in hybrid electric vehicle (PGEV)

Pure hybrid (battery plus motor/turbine – HEV)

All electric drive (battery or fuel cell)

Development is focussed mainly on the E-haulage and BEV sectors

E-haulage on the e-highway

Trucks powered by overhead wires have been used in mining for several years, but his has been a specific application with a limited circuit and slow speeds. The principle of overhead power feeds to trucks on short sections of highway is being piloted in several countries. An example is the Siemens system being piloted in both California and Sweden (Fig.1). The system is based on trucks with a hybrid electric drive which can switch from electric to diesel or natural gas power. The purpose of the pilot is to evaluate the effectiveness of a catenary system. The main application of the system is the short haul of heavy goods.

Heavy-duty trucks are the number one source of smog-forming emissions in Southern California. Developing a zero- or near-zero goods movement system in the ports will reduce smog-forming, toxic and greenhouse gas emissions in communities around the ports, which are some of the most heavily impacted by air pollution.

A battery-electric truck, a natural gas-augmented electric truck and a diesel-hybrid truck are being driven on a one-mile catenary system [1]. The interesting case here is the battery electric truck, which seems to be the solution of choice for future freight vehicles, and which allows the battery to be recharged when driving on the e-highway.

With on-board batteries added to the trucks, the company says that all of Germany’s roads could be can be outfitted for long-distance electric hauling with just 4000 km of catenary wire. Trucks would be able to recharge on highways and operate on battery power while on rural and urban streets. It is estimated that the system will be far cheaper than alternatives such as hydrogen fuel cells [5].

Hybrid systems

Hybrid powertrain systems have been slow to migrate to the HVG market but there are a number of models already in use. Hybrids are typically used for short haul trips with a large number of start stop actions which facilitate regenerative charging. Typical applications include urban delivery and utility vehicles, with single shift operation, allowing slow battery recharging in off-shift or off-peak times.

Hybrid vehicles using fuel cells are also being considered by some manufacturers. The disadvantage of the fuel cell is the response time or ramp up time from idle. Hybrid systems consisting of hydrogen fuel cells in combination with Li-ion batteries are also receiving attention. The combination allows a smaller fuel cell to be used, which charges the battery during idle or under low-load conditions. The fuel cell can be kept running and the battery provides the additional power necessary for start-up or high-load conditions.

The battery electric goods vehicle (BEV)

The difference between the battery powered passenger vehicle and the goods vehicle is the amount of space available for the battery. A goods vehicle can accommodate a larger battery, although this would reduce the hauling capability and payload. The ultimate aim is an autonomous electric drive vehicle which can haul heavy goods over long distances without needing to recharge.

Although this goal is being approached, it is far from being realised, and although heavy duty BEVs are available, their range is limited by the size and mass of the battery, as well as the absence of suitable recharge stations along the route. Factors limiting the use of BEV to urban distribution applications are battery weight and recharge times.

At the heart of the development is the Li-ion battery, in various forms, which offers higher energy and power density than lead acid batteries. Vehicles may use a single electric motor or a system of electric motors on each wheel or on each axle. This allows control of adverse situations such as jack-knifing.

Short distance haulage

Most BEVs available today are intended for urban distribution, with a daily range of less than 300 km. There are several models in production or undergoing road trials. Battery size and capacity, as well as recharge times are limiting factors with current technology for electric trucks. Table 1 lists the characteristics of some models. The range will depend on application and is assumed to be for an urban delivery, with a multiple stop-start profile. For medium to light duty trucks, the mass of the battery can be a limiting factor and could significantly reduce the payload. Fig.2 shows a typical model.

Table 1: Comparisons of various electric trucks.

Make

Models

Max. total mass (t)

Pay Load

(kg)

Battery capacity

(kWh)

Charge time (h)

Range

(km)

BYD

T7

10,5

5000

175

1,75

200

EVI

EVI-MD

10,5

–

99

6 to 12

150

Daimler

Fuso-eCanter

7,5

4600

83

7

100

Man

e-Truck

26

–

106

–

150

Mercedes Benz

Electric truck

26

–

212

–

200

Cummins

AEOS

34

–

140

1

160

Long distance freight haulage involves travelling a more or less constant speed for long periods of time without stops and starts. This affects the range of the vehicle in two ways:

No benefit from regenerative braking, which decreases capacity

Less start-up energy used, which increases capacity

The target capacity for long distance haulage requires 1000 km on a single charge. For the South African market, distances of 500 to 600 km on single charge would be acceptable, as this represents the realistic distance between large towns on a single or two day trip. Claims have been made for future all-electric battery HGVs to have ranges in excess of 1000 km

Limitations of battery driven trucks for long haul applications

The technology for longer hauls simply doesn’t exist yet. Estimations from industry show that it would require something like 8600 kg of batteries to do 1000 km on a single charge, which, from a cost and a weight perspective, does not make sense. With current technology, battery mass can be a limiting factor on the range of haulage trucks. This is probably why existing models are limited to a short range (100 to 150 km) between charges.

One of the problems with long distance vehicles is that there is a limited possibility of using regenerative braking to provide power to recharge batteries, as most long haul vehicles travel at a constant speed in what amounts to a truck lane and only make limited use of braking. The possibility of using downhill motion sounds attractive but is also not possible as the truck needs to be kept under constant control.

Many haulage vehicles are based on the semi-trailer truck principle, where the power is provided by a tractor unit to which a trailer is attached. The trailer is not permanently associated with the tractor, meaning that all the energy must come from the tractor unit, placing a restriction on the battery size, unless the trailer is also equipped with batteries.

There are several plans for long distance battery powered trucks in the pipeline.

A further problem facing long haul vehicles is the absence of charging stations with sufficient capacity. The charging rate for a long-haul vehicle would have to be of the order of 1 MW to achieve a short charging time. One manufacturer of BEVs has announced the intention of building solar-powered charging stations in the USA to meet the demand. This may limit the use of the vehicle to the more sunny parts of the country.

Fig. 2: A typical electric truck ( BYD).

The balance between battery mass and payload capacity

Vehicle energy consumption

For an electric vehicle, energy consumption is measured in kWh/100 km, and is directly proportional to speed and load. Most commercial vehicles have a limited top speed of 100 km/h. Table 2 shows the energy consumption of some electric trucks.

Table 2: Energy consumption of some BEVs.

Vehicle

Energy consumption (kWh/100 km)

Daimler FUSO -eCanter

83

BYD 5

58

BYD 9

125

Tesla Semi

120

Cummins AEOS

91

E Force one

80 to 110 (highway)

60 to 90 (urban)

The energy consumption of a truck will determine its range, with a particular battery size. Increasing the battery size will increase the range but also increase mass. Battery energy density is expressed in Wh/kg of mass. Figures for typical batteries used in EVs are given in Table 3 [4].

Table 3: Typical battery energy densities [4].

Battery type

Energy density (Wh/kg)

LiFePO4

80-120

Li CoO2

100-265

A vehicle with a payload of 20 000 kg, an energy consumption of 125kWh/100km using a LiFePO4 battery with an average capacity of 100 Wh/kg, with a mass of 1880 kg, would provide a range of 150 km. Increasing this to 600 km would increase the mass of the battery by 5640 kg and reduce the payload from 20 000 kg to 14 000 kg. Increasing the vehicle range means increasing the battery size and hence battery weight. The purpose of a HGV is haulage and any additional weight can have two possible effects:

Decreasing the payload capacity, which means lower profitability.

Requiring a larger motor for the same haulage capacity, which increases energy usage and decreases range. Hauling a larger battery with a larger motor decreases the range.

Adding a larger battery to a HGV increases its weight and reduces its haulage capacity. In addition to increased battery mass, a larger battery will require extra support and mounting, which increases the vehicle’s overall mass even further. This results in what has been called the multiplication factor (MF). MF is the factor by which the vehicle’s mass increases over the basic mass of the battery. The MF can be as high as three with current battery technology [2].

The challenge facing long distance haulage vehicles is the energy density of the battery and hence battery mass. Future Li-ion batteries using advanced technology are expected to reach figures above 500 Wh/kg [5], but this is still far in the future and electric trucks are expected to be limited to urban or short haul highway application for the foreseeable future.

Recharging stations

An urban distribution vehicle working a single shift per day would not have problems with recharging, since this could be done outside of working hours, and the recharge period is not critical. Vehicles working for longer periods would require rapid recharge facilities. Long haul vehicles would require recharge during the journey and this is a cause of concern as large batteries would require a very high rate of recharge to achieve short recharge periods. Table 4 lists possible recharge rates required for several battery sizes, ignoring recharge efficiency.

A 600 kWh battery, which could give a range of about 900 km, would require 1,2 MW of power to recharge in 30 min. A 300 kWh battery, which would give a range of about 450 km, would require 600 kW to recharge in 30 min. A truck recharge station with say 20 recharge positions, each with 500 kW capacity, would require 10 MW of peak charging power. Proposals have been made of using solar power to provide charging. If we consider that a 1 MW solar plant could generate about 6 MWh of solar energy/day, this could then charge twenty 450 km range trucks/day, which is not much.

Utility vehicles

Utility vehicles can be classed as vehicles which require power when stationary, to perform auxiliary functions, as well as for propulsion. There is a growing tendency to use electric power from batteries for auxiliary functions. Examples include trash compactors, water service trucks with pumps, electricity service vehicles, etc.